1. Field of the Invention
The present invention relates to a collision avoidance device for preventing a rendezvousing spacecraft from colliding against a target when an abnormal situation arises during the flight by employing a sensor close to a target in space, in addition to a guidance and control device, thrusters, and the like.
2. Description of the Related Art
FIG. 5 shows a collision avoidance device and the associated devices of a conventional rendezvous spacecraft. In FIG. 5, numeral 1 represents a navigation sensor (hereinafter referred to as a sensor) for measuring and computing flight conditions of the rendezvousing spacecraft, such as a relative position to a target, a posture, and acceleration. Numeral 2 represents a guidance and control device. Numeral 3 represents a circuit (hereinafter referred to as a thruster driver for ordinary use) for driving a group of thrusters (hereinafter referred to as a group of thrusters for ordinary use) which generate translation force and triaxial rotation torque in three dimensional space. Numeral 4 represents a group of thrusters for ordinary use. Numeral 5 represents a collision avoidance device. Numeral 6 represents an abnormality determining circuit. Numeral 7 represents an avoidance operation command circuit. Numeral 16 represents a circuit (hereinafter referred to as a thruster driver for redundant use) for driving a group of spare thrusters (hereinafter referred to as a group of thrusters for redundant use) which have functions equivalent to those of the group of thrusters for ordinary use. Numeral 17 represents the group of thrusters for redundant use whose constitution is same as that of the group of thrusters for ordinary use.
In the conventional arrangement described above, each of those devices, including the collision avoidance device 5, operates in the following manner. During a normal flight, in accordance with instrumentation signals transmitted from the sensor 1, the guidance and control device 2 computes required controlling force of six degrees of freedom, in total, concerning position control and triaxial posture control in three dimensional space. A drive command is then generated for each thruster. The thruster driver for ordinary use 3 drives the group of thrusters for ordinary use 4 in response to the received drive command. After execution of a sequence of the operations, a rendezvous flight to the target is performed. However, if any one of the devices malfunctions, for example, due to an abnormal operation, e.g., a cutoff of output from the sensor 1 or an erroneous injection, a normal rendezvous flight will not be performed. In a worst case, the chaser spacecraft and the target may collide with each other.
In order to avoid such a collision during the flight, in the collision avoidance device 5, the abnormality determining circuit 6 selects a signal which the guidance and control device 2 has designated from among monitor signals concerning the flight conditions, (e.g., a relative position, a posture, and acceleration), outputted by the sensor 1. The abnormality determining circuit 6 then compares the signal, or a value computed from the signal, with a reference value which has been previously set in the abnormality determining circuit 6, thereby performing a repeated determination of the presence of an abnormality, such as a mechanical failure, an excess of posture error, and a deviation from a predetermined orbit. When any abnormality is found, the avoidance operation command circuit 7 is notified of the abnormal incident. The avoidance operation command circuit 7 then determines how to cope with the situation based on the type of abnormal incident and a criterion showing the risk of collision, (e.g., an estimated amount of time the spacecraft reaches the target), which is provided by the guidance and control device 2. An outcome of the determination is transmitted to the guidance and control device 2 as a command to perform the collision avoidance.
If the avoidance operation command circuit 7 determines that any failure arises or may arise in the group of thrusters although there is not sufficient time left before the spacecraft reaches the target, the group of thrusters will be switched to the group of thrusters for redundant use, and a command to perform the injection for collision avoidance will be given to the guidance and control device 2. In accordance with the command, the guidance and control device 2 computes the required controlling force of six degrees of freedom, to carry out the injection for collision avoidance and to maintain an appropriate posture during the period, and inputs a drive command for each thruster, to the thruster driver for redundant use 16. Consequently, a group of thrusters for redundant use 17 is activated and the injection for collision avoidance is finally executed.
A correlation between a direction of the injection for collision avoidance and an avoidance orbit is shown by an "equation 1." Equation 1 is a so-called Hill equation. This is an equation of motion which shows a relative position of the rendezvousing spacecraft to the target by setting the coordinate origin to be the target, X-axis to be in the direction of the orbit of the target, and Z-axis to be in the direction of the center of the earth. (Y-axis, namely, motion in the direction which deviates from the spacecraft's orbital plane is omitted because such motion does not influence collision avoidance.) FIG. 6 illustrates an example of the solution of the equation 1. In FIG. 6, numeral 15 represents the rendezvousing spacecraft and numeral 18 represents the target. Coordinates X and Z are in the direction of the orbit of the target and the direction of the center of the earth, respectively, as shown in the equation 1. As shown in the drawing, if thrust is applied in the direction of orbit by the injection for collision avoidance, the rendezvousing spacecraft 15 will pass over and move away in the backward direction towards the target 18. If the thrust is applied in the reverse direction of the orbit, the rendezvousing spacecraft 15 will pass below and move away in the forward direction towards the target 18.
Equation 1! EQU d.sup.2 X/dt.sup.2 -2.omega.dZ/dt=0 EQU d.sup.2 Z/dt.sup.2 +2.omega.dX/dt-3.omega..sup.2 Z=0
X: Relative distance of the rendezvousing spacecraft, which is moving in the forward direction, to the target PA1 Z: Relative distance of the rendezvousing spacecraft, which is moving in the direction of the earth, to the target PA1 .omega.: Orbit angular velocity
The aforementioned principle is applied to collision avoidance. In order to execute collision avoidance safely, a relative position to the target, and a direction and time period of the injection according to relative velocity, shall be appropriately set beforehand in the avoidance operation command circuit 7. In order to avoid such a collision, it is essential to apply the thrust as previously set. Therefore, the rendezvousing spacecraft is usually equipped with at least a group of thrusters for redundant use 17 as a way of precaution against a failure of the group of thrusters for ordinary use 4. An example of the arrangement of the groups of thrusters is shown in FIG. 7. As shown in FIG. 7, for the purposes of generating three dimensional translation force and triaxial rotation torque, the arrangement of a group of thrusters for rendezvousing is such that a mass of thrusters are arranged in a complicated manner. Here, a solid line arrow represents one of the thrusters for avoidance, and a dashed line arrow represents one of the thrusters for redundant use. Each of these arrows shows a direction of the injection.
There is another method of preventing such a collision. In this method (hereinafter referred to as an arrangement method), none of the thrusters for redundant use 17 are used, but translation force and rotation torque for the collision avoidance are generated by removing one or more of the broken thrusters from the group of thrusters for ordinary use 4 and then combining the remaining thrusters. This is a method which uses software logic capable of processing according to the number of combination of thrusters shown in an "equation 2." More specifically, in the guidance and control device 2 of FIG. 5, the software logic needed to generate the required translation force and rotation torque is prepared in advance with arbitrary thrusters being excluded, and a command to perform posture control and injection for collision avoidance is provided to the thruster driver for ordinary use 3. Here, the posture control and the injection for collision avoidance are performed by a combination of thrusters which excludes the broken thruster(s) specified by the collision avoidance device 5.
Equation 2!
The number of combination(C) when excluding m pieces from n pieces of thrusters: ##EQU1##
Example of calculation: if n is 16 and m is 2, .sub.16 C.sub.2 will be 120 ways.
In the prior art, there are some problems. More specifically, the injection for collision avoidance and the posture control during the injection are carried out by using the group of thrusters for redundant use 17, which function to generate three dimensional translation force and triaxial rotation torque, equivalent to the group of thrusters for ordinary use as described above. Consequently, the prior art has superfluous functions for ensuring safety of the rendezvousing spacecraft and the target and involves a complicated construction and a comparatively high cost. On the other hand, if such injection for collision avoidance and posture control during the injection are carried out, for the purposes of solving the aforementioned problems, by an arbitrary combination of the remaining thrusters which are left after only arbitrary thrusters, namely, broken thrusters were excluded from a group of thrusters for ordinary use, there are a larger number of ways of combining these thrusters as shown in the equation 2. Therefore, software for processing the data will become large in scale, and costs required for the verification of the software will increase.